1. Field
The present invention relates to the field of DSL (Digital Subscriber Line) communications receivers, and in particular to filtering noise out of DSL data signals using a reference signal.
2. Related Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines (referred to as loops and/or the copper plant). Telephone subscriber lines can provide this bandwidth despite their original design for only voice-band analog communication. In particular, asymmetric DSL (ADSL) and very-high-speed DSL (VDSL) can adapt to the characteristics of the subscriber line by using a discrete multitone (DMT) line code that assigns a number of bits to each tone (or sub-carrier), which can be adjusted to channel conditions determined during initialization and subsequent on-line training known as “bit-swapping” of the modems (typically transceivers that function as both transmitters and receivers) at each end of the subscriber line.
ADSL service uses frequencies in the range of 138 KHz to 1.1 MHz for operation. Nearly 5,000 AM (Amplitude Modulation) radio stations in the United States use frequencies in the range of 540 KHz to 1.7 MHz. These radio signals permeate many areas, including areas in which users have DSL modems in operation. The sizable overlap in frequencies usage can create problems for DSL users. In addition, other sources of radio frequency (RF) interference can contribute to a deterioration in DSL system performance as a result of the interference they cause. Finally, other types of interference also can interfere with data signals sent on DSL and other communication systems, such as crosstalk, impulse noise, power-line noise, and other man-made electronic radiation.
RF interference does not distort the entire spectrum identified above. Instead, many sources, such as AM radio stations, affect only a very narrow portion of the frequency spectrum. ADSL uses 128 or 256 carriers, each of which is a discrete segment of the frequency spectrum about 4.3125 kHz wide. Because the ADSL system blocks the transmissions into packets or symbols of information that are 250 microseconds in length, there is a windowing effect that causes the receiver to see RF interference within tens to hundreds of kilohertz of the center of each and every carrier used in the ADSL system. Theoretically, 5 KHz wide RF AM radio interference would tend to affect only 2-3 ADSL carriers, but the windowing effect leads to each AM radio station possibly affecting anywhere from several to tens of carriers. Impulse, power-line and other noise sources can often affect a wide range of frequencies.
In many prior systems, the modem affected by RF interference and other noise sources at a given carrier merely stops using the affected carriers or at least reduces the number of bits that the modem carries in the vicinity of the RF interference, which lowers the performance of the DSL system. The effect is especially pronounced when the interference is present at the end of a long DSL line. Signals that have been attenuated significantly during transmission can be completely overcome by RF interference at a customer's premises. While twisting of the transmission loop wires mitigates some of the ingress of RF interference, it nevertheless represents a significant problem. As the frequency band used by the DSL system increases (for example, ADSL2+, VDSL), the twisting or balance of the twisted pair becomes less effective so that the higher the frequency of the RF ingress, the larger its coupling into the pairs. Furthermore, higher frequencies on a twisted pair tend to be the most attenuated, and so are more susceptible to distortion by crosstalk at higher frequencies.
In particular, noise and interference often couples most strongly to telephone lines between customers' premises and pedestals (service terminals) and the like. Pedestals offer a cross-connection point between lines going from a central office (CO) (or remote terminal central office) to a specific customer premises or a few customer premises (often referred to as a “drop”). The remainder of lines from the CO may continue to other pedestals. Typically, there are 2-6 lines in the “drop” segment to each customer, providing extra copper for the contingency of one or more customers later demanding multiple phone services. The relatively exposed DSL transmission loop segment running between the pedestal and customer premises acts as an antenna, picking up the noise and interference signals, including AM radio broadcasts in the area. This segment of the line may experience vertical runs of the line that tend to act as higher gain antennas to the RF signals and other noise. Additionally, this last segment is often not well shielded or employs shields that are not well grounded, leading to additional gain in receipt of noise and interference by the telephone line(s).